Found the game... Metal Masters...
I played a game at a friends house a long time ago. I am not sure, but it could have been on his Atari ST.
What I remember is that you fought with giant robots. First you had to choose parts and build a robot. You could put hammers as fists and more.
I am not sure, but I can imagine that you actually fought in some kind of league and could upgrade your robot along the way after you won some matches and so on. (Not sure about that though.)
EDIT---
Oh, and it's a 2D-game. You see the robots from the side.
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Anyone who remember this ones name? I have no clue at all, but I will probably recognize it when I see some screenshots.
Thanks!